Bump WebKit (oven-sh/WebKit#650 preview): (a?.b)() throws for a nullish a, and a class field makes the call

[robobun]

Problem

• (a?.b)() with a nullish a returns undefined. The spec and node run the arguments, then throw a TypeError.
• In a class field, g = (o?.f)() with a present o stores [Function: f] and never calls it. TypeScript (o?.f as F)() and (o?.f)!() print as this shape.
• Cause, in JavaScriptCore: ASTBuilder::makeFunctionCallNode (parser/ASTBuilder.h) puts the call back inside the OptionalChainNode of a?.b. The short-circuit jump then skips the call. That node also starts at a, so the second parse of a field initializer stops at the ).

Fix

• [JSC] (a?.b)(): a nullish a throws a TypeError, and a class field initializer makes the call WebKit#650 ends the optional chain at the callee. A nullish base loads undefined into the callee and this, the arguments run, and the call throws (o?.a) is not a function. A present base is still the this.
• Correct because parentheses end an optional chain, and a short-circuited chain is the plain value undefined. a?.b() and (a?.b)?.() stay one chain each.
• Pins the preview build of that PR. Before this lands, [JSC] (a?.b)(): a nullish a throws a TypeError, and a class field initializer makes the call WebKit#650 must merge and WEBKIT_VERSION must move to the merged sha. Self-reviewed: 5 concerns raised, 3 addressed (Notes).
• Verified: the new fixture optional-chaining-parenthesized-call.js in test/js/bun/jsc-stress/jsc-stress.test.ts, and a new test in test/bundler/transpiler/runtime-transpiler.test.ts for the TypeScript spellings. Both fail on the cf1b36ec pin and pass on the preview.

Background

• Bun links a prebuilt JavaScriptCore from oven-sh/WebKit, pinned in scripts/build/deps/webkit.ts. A preview tag lets a bump PR test a WebKit PR before it merges.
• test/js/bun/jsc-stress/ runs WebKit JSTests/stress files under bun. Their // @bun line skips the transpiler.
• Bun's transpiler keeps these parentheses, so every file reaches the engine with them.

Notes

Repro (bun 1.4.3 canary, WebKit cf1b36ec):

const t = (src) => { try { return String(new Function("o", "return " + src)(undefined)); } catch (e) { return "throws " + e.constructor.name; } };
for (const s of ["(o?.a)()", "(o?.a.b)()", "(o?.[0])()", "((o?.a))()", "(o?.a)``", "(o?.a).b", "(0, o?.a)()", "o?.a()", "(o?.a)?.()"]) console.log(s.padEnd(14), "=>", t(s));

node prints throws TypeError for the first seven and undefined for the last two. bun printed undefined for the first four.

• (a?.b.c)(), (a?.[k])() and ((a?.b))() behave like (a?.b)().
• Nobody reported either symptom. Both came up in tests of the decorator lowering (js_parser: lower standard decorators without moving class members #40833).
• test/bundler/bundler_bytecode_portable.test.ts keeps its snapshot: no corpus file has this shape.
• The WebKit branch sits on cf1b36ec, the old pin. So the preview autobuild-preview-pr-650-85a710c2 differs from the old pin by the one commit of [JSC] (a?.b)(): a nullish a throws a TypeError, and a class field initializer makes the call WebKit#650.
• preload.js gets drainMicrotasks, the way the jsc shell settles promises. Bump WebKit (oven-sh/WebKit#534 preview): call the tag of (a?.b)... with this = a #40848 adds the same three lines.
• The full message is (o?.a) is not a function. (In '(o?.a)()', '(o?.a)' is undefined), the message of any other call of undefined.
• Also ran on the debug build with the preview: all 117 tests of jsc-stress.test.ts, test/js/bun/jsc/bun-jsc.test.ts, test/js/bun/jsc/webkit-upgrade-6b879687ee.test.ts and test/js/node/vm/vm.test.ts.
• Self-review: it asked for a test of the class field case, for a title and body that name that case and its TypeScript spellings, and for the delete note below. All three are in [JSC] (a?.b)(): a nullish a throws a TypeError, and a class field initializer makes the call WebKit#650 and here. It also asked for a bugs.webkit.org report with an upstream patch. That is still to do, and [JSC] (a?.b)(): a nullish a throws a TypeError, and a class field initializer makes the call WebKit#650 says so. It proposed to fold this bump into Bump WebKit (oven-sh/WebKit#534 preview): call the tag of (a?.b)... with this = a #40848. I did not: each WebKit PR has its own preview bump PR in this repo, and the two WebKit changes touch different functions.
• Found next to it, not fixed here: class G { y = delete o?.g } gives y the value of o?.g and deletes nothing. makeDeleteNode returns the inner OptionalChainNode, which starts after the delete keyword, and the second parse of the field initializer starts at the position of that node.

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[policy-decision:webkit] gate passed · iteration 0 · 5 files touched

passes on PR (with fix)

Test-only change.

Debug/ASAN (expected pass):
$ bun bd test 'test/bundler/transpiler/runtime-transpiler.test.ts' 'test/js/bun/jsc-stress/jsc-stress.test.ts'
$ BUN_DEBUG_QUIET_LOGS=1 bun scripts/build.ts --profile=debug --quiet test test/bundler/transpiler/runtime-transpiler.test.ts test/js/bun/jsc-stress/jsc-stress.test.ts
bun test v1.4.3 (b99371011)

test/bundler/transpiler/runtime-transpiler.test.ts:
(pass) use strict causes CommonJS [462.42ms]
(pass) non-ascii regexp literals [6.72ms]
(pass) ascii regex with escapes [2.77ms]
(pass) // @bun > async transpiler [39.99ms]
(pass) // @bun > require() [23.77ms]
(pass) // @bun > synchronous [467.51ms]
(pass) json imports > require(*.json) [28.80ms]
(pass) json imports > import(*.json) [14.14ms]
(pass) json imports > should support comments in tsconfig.json [10.45ms]
(pass) json imports > should handle non-boecjts in tsconfig.json [10.35ms]
(pass) json imports > should handle duplicate keys [7.86ms]
(pass) with statement > works [385.95ms]
(pass) math.pow [8.27ms]
(pass) unterminated string literals in large files > reports an unterminated string literal at the end of a large JavaScript file [544.61ms]
(pass) unterminated string literals in large files > reports an unterminated string literal at the end of a large JSON file [391.84ms]
(pass) the call of a parenthesized optional chain is not part of the chain [401.84ms]

test/js/bun/jsc-stress/jsc-stress.test.ts:
(pass) JSC JIT Stress Tests > JS (Baseline/DFG/FTL) > ftl-arithtan.js [400.13ms]
(pass) JSC JIT Stress Tests > JS (Baseline/DFG/FTL) > ftl-arithsin.js [479.45ms]
(pass) JSC JIT Stress Tests > JS (Baseline/DFG/FTL) > ftl-arithcos.js [458.24ms]
(pass) JSC JIT Stress Tests > JS (Baseline/DFG/FTL) > ftl-arithsqrt.js [457.72ms]
(pass) JSC JIT Stress Tests > JS (Baseline/DFG/FTL) > ftl-string-equality.js [464.83ms]
(pass) JSC JIT Stress Tests > JS (Baseline/DFG/FTL) > ftl-library-substring.js [427.66ms]
(pass) JSC JIT Stress Tests > JS (Baseline/DFG/FTL) > ftl-string-strict-equality.js [467.01ms]
(pass) JSC JIT Stress Tests > JS (Baseline/DFG/FTL) > ftl-regexp-exec.js [463.90ms]
(pass) JSC JIT Stress Tests >
... (truncated)
Exit: 0

diff hotspot

scripts/build/deps/webkit.ts                       |   2 +-
 test/bundler/transpiler/runtime-transpiler.test.ts |  56 +++
 .../optional-chaining-parenthesized-call.js        | 514 +++++++++++++++++++++
 test/js/bun/jsc-stress/jsc-stress.test.ts          |   2 +
 test/js/bun/jsc-stress/preload.js                  |   3 +
 5 files changed, 576 insertions(+), 1 deletion(-)

gate history · 3 passed · 0 rejected · iteration 0

evidence per changed file

file                                                      reads  edits  tests
scripts/build/deps/webkit.ts                                  3      3     23
test/bundler/transpiler/runtime-transpiler.test.ts            1      0     15
…stress/fixtures/optional-chaining-parenthesized-call.js      0      0     25
test/js/bun/jsc-stress/jsc-stress.test.ts                     1      1     14
test/js/bun/jsc-stress/preload.js                             1      1     23

[robobun]

Status: the engine fix is oven-sh/WebKit#650. This PR pins its preview build autobuild-preview-pr-650-85a710c2.

How I reproduced it, on bun 1.4.3 canary (WebKit cf1b36ec) and on the jsc shell of that pin:

const missing = undefined;
console.log((missing?.a)());           // bun: undefined         node: TypeError

const o = { f() { return "called"; } };
class F { g = (o?.f)(); }
console.log(new F().g);                // bun: [Function: f]     node: "called"

Both new tests fail with USE_SYSTEM_BUN=1 and pass with bun bd on the preview:

• bun bd test test/js/bun/jsc-stress/jsc-stress.test.ts -t optional-chaining-parenthesized-call
• bun bd test test/bundler/transpiler/runtime-transpiler.test.ts -t "parenthesized optional chain"

The preview is the old pin cf1b36ec plus the one commit of oven-sh/WebKit#650.

This PR is a draft because it cannot land on the preview tag. When oven-sh/WebKit#650 merges, WEBKIT_VERSION moves to the merged sha and the PR becomes ready.

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Walkthrough

The PR selects a WebKit preview release and adds regression coverage for parenthesized optional-chain calls. The coverage includes TypeScript transpilation, JSC stress execution, receiver binding, argument evaluation, class fields, optimization paths, and asynchronous completion.

Changes

Optional chaining regression coverage

Layer / File(s)	Summary
WebKit preview selection scripts/build/deps/webkit.ts	WEBKIT_VERSION now uses the autobuild-preview-pr-650-13bd3c9b release identifier.
Transpiler regression coverage test/bundler/transpiler/runtime-transpiler.test.ts	Adds TypeScript integration coverage for parenthesized optional-chain calls, including casts, non-null assertions, receiver binding, argument evaluation, and nullish failures.
JSC stress fixture coverage test/js/bun/jsc-stress/fixtures/optional-chaining-parenthesized-call.js	Adds stress coverage for optional-chain calls across syntax forms, receiver types, class fields, private names, optimization paths, and asynchronous execution.
Stress fixture registration test/js/bun/jsc-stress/jsc-stress.test.ts, test/js/bun/jsc-stress/preload.js	Registers the fixture under the Parser/bytecode generator category and adds globalThis.drainMicrotasks using jsc.drainMicrotasks.

Priority: ➖ Normal

Severity of issue fixed: Medium

Merge Risk: 🟡 Moderate · up to b7c76

The WebKit dependency must be updated to the merged upstream commit before this change can safely land.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The PR satisfies #650. WEBKIT_VERSION pins autobuild-preview-pr-650-13bd3c9b before the upstream fix merges. The new JSC stress fixture covers nullish and present bases, this preservation, argum…
Out of Scope Changes check	✅ Passed	The changes remain within #650. The dependency pin supplies the engine fix. The stress fixture and transpiler test validate the linked behavior. The drainMicrotasks addition supports asynchronous st…
Title check	✅ Passed	The title clearly identifies the WebKit preview bump and the primary parenthesized optional-chain call fix. It is specific and related to the main changes.
Description check	✅ Passed	The description explains the problem, fix, affected behavior, verification steps, test coverage, and preview-build dependency. It does not use the template headings exactly, but it provides the requir…

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Actionable comments posted: 2

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Inline comments:
In `@scripts/build/deps/webkit.ts`:
- Line 6: Update WEBKIT_VERSION after WebKit#650 merges to the corresponding
merged autobuild-<sha> release tag, replacing the temporary
autobuild-preview-pr-650 pin; leave the existing assertion unchanged.

In `@test/bundler/transpiler/runtime-transpiler.test.ts`:
- Around line 259-297: Update the single-file transpiler fixture in the test
around the Fields class and optional-chain calls to run inline via bunExe() with
the -e flag instead of creating a tempDir. Preserve the existing TypeScript
source and execution assertions, and keep tempDir only for fixtures requiring
multiple files.

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• scripts/build/deps/webkit.ts
• test/bundler/transpiler/runtime-transpiler.test.ts
• test/js/bun/jsc-stress/fixtures/optional-chaining-parenthesized-call.js
• test/js/bun/jsc-stress/jsc-stress.test.ts
• test/js/bun/jsc-stress/preload.js

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[robobun]

^{Updated 12:56 AM PT - Sep 14th, 2026}

✅ @robobun, your commit e97fb562d3ea6492d8bd0446f641adc8d8d25d9d passed in Build #115433! 🎉

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🧪   To try this PR locally:

bunx bun-pr 42684

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bun-42684 --bun

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